The Role of Ethylene Production in Virulence of Pseudomonas syringae pvs. glycinea and phaseolicola

Substitution of 2-oxoglutarate alters reaction outcomes of the Pseudomonas savastanoi ethylene-forming enzyme

In seeding plants, biosynthesis of the phytohormone ethylene, which regulates processes including fruit ripening and senescence, is catalyzed by 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase. The plant pathogen Pseudomonas savastanoi (previously classified as: Pseudomonas syringae) employs a different type of ethylene-forming enzyme (psEFE), though from the same structural superfamily as ACC oxidase, to catalyze ethylene formation from 2-oxoglutarate (2OG) in an arginine dependent manner. psEFE also catalyzes the more typical oxidation of arginine to give L-Δ1-pyrroline-5-carboxylate (P5C), a reaction coupled to oxidative decarboxylation of 2OG giving succinate and CO2. We report on the effects of C3 and/or C4 substituted 2OG derivatives on the reaction modes of psEFE. 1H NMR assays, including using the pure shift method, reveal that, within our limits of detection, none of the tested 2OG derivatives is converted to an alkene; some are converted to the corresponding β-hydroxypropionate or succinate derivatives, with only the latter being coupled to arginine oxidation. The NMR results reveal that the nature of 2OG derivatization can affect the outcome of the bifurcating reaction, with some 2OG derivatives exclusively favoring the arginine oxidation pathway. Given that some of the tested 2OG derivatives are natural products, the results are of potential biological relevance. There are also opportunities for therapeutic or biocatalytic regulation of the outcomes of reactions catalyzed by 2OG-dependent oxygenases by the use of 2OG derivatives.

Bio-Polyethylene and Polyethylene Biocomposites: An Alternative toward a Sustainable Future

Polyethylene (PE), a highly prevalent non-biodegradable polymer in the field of plastics, presents a waste management issue. To alleviate this issue, bio-based PE (bio-PE), derived from renewable resources like corn and sugarcane, offers an environmentally friendly alternative. This review discusses various production methods of bio-PE, including fermentation, gasification, and catalytic conversion of biomass. Interestingly, the bio-PE production volumes and market are expanding due to the growing environmental concerns and regulatory pressures. Additionally, the production of PE and bio-PE biocomposites using agricultural waste as filler materials, highlights the growing demand for sustainable alternatives to conventional plastics. According to previous studies, addition of ≈50% defibrillated corn and abaca fibers into bio-PE matrix and a compatibilizer, results in the highest Young's modulus of 4.61 and 5.81 GPa, respectively. These biocomposites have potential applications in automotive, building construction, and furniture industries. Moreover, the advancement made in abiotic and biotic degradation of PE and PE biocomposites is elucidated to address their environmental impacts. Finally, the paper concludes with insights into the opportunities, challenges, and future perspectives in the sustainable production and utilization of PE and bio-PE biocomposites. In summary, production of PE and bio-PE biocomposites can contribute to a cleaner and sustainable future.

Recent Advances in Dissecting the Function of Ethylene in Interaction between Host and Pathogen

Pathogens influence the growth and development of plants, resulting in detrimental damage to their yields and quality. Ethylene, a gaseous phytohormone, serves a pivotal function in modulating diverse physiological processes in plants, including defense mechanisms against pathogen invasion. Ethylene biosynthesis is involved in both plants and pathogens. Recent empirical research elucidates the intricate interactions and regulatory mechanisms between ethylene and pathogens across various plant species. In this review, we provide a comprehensive overview of the latest findings concerning ethylene's role and its regulatory networks in host-pathogen interactions. Additionally, we explore the crosstalk between ethylene and other phytohormones. Points regarding ethylene emission and its modulation by pathogens are also emphasized. Moreover, we also discuss potential unresolved issues in the field that warrant further investigation.

Biological formation of ethylene

This review summarizes the structures, biochemical properties, and mechanisms of two major biological sources of ethylene, the ethylene-forming enzyme (EFE) and 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACCO). EFE is found in selected bacteria and fungi where it catalyzes two reactions: (1) the oxygen-dependent conversion of 2-oxoglutarate (2OG) to ethylene plus three molecules of CO2/bicarbonate and (2) the oxidative decarboxylation of 2OG while transforming l-arginine to guanidine and l-Δ1-pyrroline-5-carboxylic acid. ACCO is present in plants where it makes the plant hormone by transforming ACC, O2, and an external reductant to ethylene, HCN, CO2, and water. Despite catalyzing distinct chemical reactions, EFE and ACCO are related in sequence and structure, and both enzymes require Fe(ii) for their activity. Advances in our understanding of EFE, derived from both experimental and computational approaches, have clarified how this enzyme catalyzes its dual reactions. Drawing on the published mechanistic studies of ACCO and noting the parallels between this enzyme and EFE, we propose a novel reaction mechanism for ACCO.

Deciphering host-pathogen interaction during Streptomyces spp. infestation of potato

Potato crop, currently, is the staple food crop of about 1.3 billion global population. Potato is attaining even more admiration globally day by day owing to its public acceptability. However, potato sustainable production is distinctly challenged by multiple factors like diseases, pests and climate change etc. Among diseases, common scab is one of the prime threats to potato crop due to its soil-borne nature and versatility in phytotoxins' secretion. Common scab is caused multiple number of phytopathogenic streptomyces strains. Despite extensive research programs, researchers are still unable to identify a significant solution to this threat that is proliferating exceptional rate across the globe. To develop feasible remedies, adequate information regarding host-pathogen interaction should be available. This review possesses insights on existing pathogenic species, the evolution of novel pathogenic streptomyces spp. and phytotoxins produced by the pathogenic strains. Furthermore, which type of physiological, biochemical and genetic activities occur during pathogen's infestation of the host are also canvassed.

A tale of two lineages: how the strains of the earliest divergent symbiotic Frankia clade spread over the world

It is currently assumed that around 100 million years ago, the common ancestor to the Fabales, Fagales, Rosales and Cucurbitales in Gondwana, developed a root nodule symbiosis with a nitrogen-fixing bacterium. The symbiotic trait evolved first in Frankia cluster-2; thus, strains belonging to this cluster are the best extant representatives of this original symbiont. Most cluster-2 strains could not be cultured to date, except for Frankia coriariae, and therefore many aspects of the symbiosis are still elusive. Based on phylogenetics of cluster-2 metagenome-assembled genomes (MAGs), it has been shown that the genomes of strains originating in Eurasia are highly conserved. These MAGs are more closely related to Frankia cluster-2 in North America than to the single genome available thus far from the southern hemisphere, i.e., from Papua New Guinea.

To unravel more biodiversity within Frankia cluster-2 and predict routes of dispersal from Gondwana, we sequenced and analysed the MAGs of Frankia cluster-2 from Coriaria japonica and Coriaria intermedia growing in Japan, Taiwan and the Philippines. Phylogenetic analyses indicate there is a clear split within Frankia cluster-2, separating a continental from an island lineage. Presumably, these lineages already diverged in Gondwana.

Based on fossil data on the host plants, we propose that these two lineages dispersed via at least two routes. While the continental lineage reached Eurasia together with their host plants via the Indian subcontinent, the island lineage spread towards Japan with an unknown host plant.

Discovery of Five New Ethylene-Forming Enzymes for Clean Production of Ethylene in E. coli

Ethylene is an essential platform chemical with a conjugated double bond, which can produce many secondary chemical products through copolymerisation. At present, ethylene production is mainly from petroleum fractionation and cracking, which are unsustainable in the long term, and harmful to our environment. Therefore, a hot research field is seeking a cleaner method for ethylene production. Based on the model ethylene-forming enzyme (Efe) AAD16440.1 (6vp4.1.A) from Pseudomonas syringae pv. phaseolicol, we evaluated five putative Efe protein sequences using the data derived from phylogenetic analyses and the conservation of their catalytic structures. Then, pBAD expression frameworks were constructed, and relevant enzymes were expressed in E. coli BL21. Finally, enzymatic activity in vitro and in vivo was detected to demonstrate their catalytic activity. Our results show that the activity in vitro measured by the conversion of α-ketoglutarate was from 0.21-0.72 μmol ethylene/mg/min, which varied across the temperatures. In cells, the activity of the new Efes was 12.28-147.43 μmol/gDCW/h (DCW, dry cellular weight). Both results prove that all the five putative Efes could produce ethylene.

Action mechanisms of small microbial volatile compounds in plants

Microorganisms communicate with plants by exchanging chemical signals throughout the phytosphere. Before direct contact with plants occurs, beneficial microorganisms emit a plethora of volatile compounds that promote plant growth and photosynthesis as well as developmental, metabolic, transcriptional, and proteomic changes in plants. These compounds can also induce systemic drought tolerance and improve water and nutrient acquisition. Recent studies have shown that this capacity is not restricted to beneficial microbes; it also extends to phytopathogens. Plant responses to microbial volatile compounds have frequently been associated with volatile organic compounds with molecular masses ranging between ~ 45Da and 300Da. However, microorganisms also release a limited number of volatile compounds with molecular masses of less than ~45Da that react with proteins and/or act as signaling molecules. Some of these compounds promote photosynthesis and growth when exogenously applied in low concentrations. Recently, evidence has shown that small volatile compounds are important determinants of plant responses to microbial volatile emissions. However, the regulatory mechanisms involved in these responses remain poorly understood. This review summarizes current knowledge of biochemical and molecular mechanisms involved in plant growth, development, and metabolic responses to small microbial volatile compounds.

Growth modulation and metabolic responses of Ganoderma boninense to salicylic acid stress

Various phenolic compounds have been screened against Ganoderma boninense, the fungal pathogen causing basal stem rot in oil palms. In this study, we focused on the effects of salicylic acid (SA) on the growth of three G. boninense isolates with different levels of aggressiveness. In addition, study on untargeted metabolite profiling was conducted to investigate the metabolomic responses of G. boninense towards salicylic acid. The inhibitory effects of salicylic acid were both concentration- (P < 0.001) and isolate-dependent (P < 0.001). Also, growth-promoting effect was observed in one of the isolates at low concentrations of salicylic acid where it could have been utilized by G. boninense as a source of carbon and energy. Besides, adaptation towards salicylic acid treatment was evident in this study for all isolates, particularly at high concentrations. In other words, inhibitory effect of salicylic acid treatment on the fungal growth declined over time. In terms of metabolomics response to salicylic acid treatment, G. boninense produced several metabolites such as coumarin and azatyrosine, which suggests that salicylic acid modulates the developmental switch in G. boninense towards the defense mode for its survival. Furthermore, the liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) analysis showed that the growth of G. boninense on potato dextrose agar involved at least four metabolic pathways: amino acid metabolism, lipid pathway, tryptophan pathway and phenylalanine pathway. Overall, there were 17 metabolites that contributed to treatment separation, each with P<0.005. The release of several antimicrobial metabolites such as eudistomin I may enhance G. boninense's competitiveness against other microorganisms during colonisation. Our findings demonstrated the metabolic versatility of G. boninense towards changes in carbon sources and stress factors. G. boninense was shown to be capable of responding to salicylic acid treatment by switching its developmental stage.

Photoautotrophic production of renewable ethylene by engineered cyanobacteria: Steering the cell metabolism towards biotechnological use

Ethylene is a volatile hydrocarbon with a massive global market in the plastic industry. The ethylene now used for commercial applications is produced exclusively from nonrenewable petroleum sources, while competitive biotechnological production systems do not yet exist. This review focuses on the currently developed photoautotrophic bioproduction strategies that enable direct solar-driven conversion of CO₂ into ethylene, based on the use of genetically engineered photosynthetic cyanobacteria expressing heterologous ethylene forming enzyme (EFE) from Pseudomonas syringae. The emphasis is on the different engineering strategies to express EFE and to direct the cellular carbon flux towards the primary metabolite 2-oxoglutarate, highlighting associated metabolic constraints, and technical considerations on cultivation strategies and conditional parameters. While the research field has progressed towards more robust strains with better production profiles, and deeper understanding of the associated metabolic limitations, it is clear that there is room for significant improvement to reach industrial relevance. At the same time, existing information and the development of synthetic biology tools for engineering cyanobacteria open new possibilities for improving the prospects for the sustainable production of renewable ethylene.

Bacterial volatile compound-based tools for crop management and quality

Bacteria produce a huge diversity of metabolites, many of which mediate ecological relations. Among these, volatile compounds cause broad-range effects at low doses and, therefore, may be exploited for plant defence strategies and agricultural production, but such applications are still in their early development. Here, we review the latest technologies involving the use of bacterial volatile compounds for phytosanitary inspection, biological control, plant growth promotion, and crop quality. We highlight a variety of effects with a potential applicative interest, based on either live biocontrol and/or biostimulant agents, or the isolated metabolites responsible for the interaction with hosts or competitors. Future agricultural technologies may benefit from the development of new analytical tools to understand bacterial interactions with the environment.

Comparative Genomics of Potato Common Scab-Causing Streptomyces spp. Displaying Varying Virulence

Common scab of potato causes important economic losses worldwide following the development of necrotic lesions on tubers. In this study, the genomes of 14 prevalent scab-causing Streptomyces spp. isolated from Prince Edward Island, one of the most important Canadian potato production areas, were sequenced and annotated. Their phylogenomic affiliation was determined, their pan-genome was characterized, and pathogenic determinants involved in their virulence, ranging from weak to aggressive, were compared. 13 out of 14 strains clustered with Streptomyces scabiei, while the last strain clustered with Streptomyces acidiscabies. The toxicogenic and colonization genomic regions were compared, and while some atypical gene organizations were observed, no clear correlation with virulence was observed. The production of the phytotoxin thaxtomin A was also quantified and again, contrary to previous reports in the literature, no clear correlation was found between the amount of thaxtomin A secreted, and the virulence observed. Although no significant differences were observed when comparing the presence/absence of the main virulence factors among the strains of S. scabiei, a distinct profile was observed for S. acidiscabies. Several mutations predicted to affect the functionality of some virulence factors were identified, including one in the bldA gene that correlates with the absence of thaxtomin A production despite the presence of the corresponding biosynthetic gene cluster in S. scabiei LBUM 1485. These novel findings obtained using a large number of scab-causing Streptomyces strains are challenging some assumptions made so far on Streptomyces’ virulence and suggest that other factors, yet to be characterized, are also key contributors.

A Breach in Plant Defences: Pseudomonas syringae pv. actinidiae Targets Ethylene Signalling to Overcome Actinidia chinensis Pathogen Responses

Ethylene interacts with other plant hormones to modulate many aspects of plant metabolism, including defence and stomata regulation. Therefore, its manipulation may allow plant pathogens to overcome the host’s immune responses. This work investigates the role of ethylene as a virulence factor for Pseudomonas syringae pv. actinidiae (Psa), the aetiological agent of the bacterial canker of kiwifruit. The pandemic, highly virulent biovar of this pathogen produces ethylene, whereas the biovars isolated in Japan and Korea do not. Ethylene production is modulated in planta by light/dark cycle. Exogenous ethylene application stimulates bacterial virulence, and restricts or increases host colonisation if performed before or after inoculation, respectively. The deletion of a gene, unrelated to known bacterial biosynthetic pathways and putatively encoding for an oxidoreductase, abolishes ethylene production and reduces the pathogen growth rate in planta. Ethylene production by Psa may be a recently and independently evolved virulence trait in the arms race against the host. Plant- and pathogen-derived ethylene may concur in the activation/suppression of immune responses, in the chemotaxis toward a suitable entry point, or in the endophytic colonisation.

Hormones as go-betweens in plant microbiome assembly

The interaction of plants with complex microbial communities is the result of co-evolution over millions of years and contributed to plant transition and adaptation to land. The ability of plants to be an essential part of complex and highly dynamic ecosystems is dependent on their interaction with diverse microbial communities. Plant microbiota can support, and even enable, the diverse functions of plants and are crucial in sustaining plant fitness under often rapidly changing environments. The composition and diversity of microbiota differs between plant and soil compartments. It indicates that microbial communities in these compartments are not static but are adjusted by the environment as well as inter-microbial and plant-microbe communication. Hormones take a crucial role in contributing to the assembly of plant microbiomes, and plants and microbes often employ the same hormones with completely different intentions. Here, the function of hormones as go-betweens between plants and microbes to influence the shape of plant microbial communities is discussed. The versatility of plant and microbe-derived hormones essentially contributes to the creation of habitats that are the origin of diversity and, thus, multifunctionality of plants, their microbiota and ultimately ecosystems.

Functional Genomics Insights Into the Pathogenicity, Habitat Fitness, and Mechanisms Modifying Plant Development of Rhodococcus sp. PBTS1 and PBTS2

Pistachio Bushy Top Syndrome (PBTS) is a recently emerged disease that has strongly impacted the pistachio industry in California, Arizona, and New Mexico. The disease is caused by two bacteria, designated PBTS1 that is related to Rhodococcus corynebacterioides and PBTS2 that belongs to the species R. fascians. Here, we assessed the pathogenic character of the causative agents and examined their chromosomal sequences to predict the presence of particular functions that might contribute to the observed co-occurrence and their effect on plant hosts. In diverse assays, we confirmed the pathogenicity of the strains on "UCB-1" pistachio rootstock and showed that they can also impact the development of tobacco species, but concurrently inconsistencies in the ability to induce symptoms were revealed. We additionally evidence that fas genes are present only in a subpopulation of pure PBTS1 and PBTS2 cultures after growth on synthetic media, that these genes are easily lost upon cultivation in rich media, and that they are enriched for in an in planta environment. Analysis of the chromosomal sequences indicated that PBTS1 and PBTS2 might have complementary activities that would support niche partitioning. Growth experiments showed that the nutrient utilization pattern of both PBTS bacteria was not identical, thus avoiding co-inhabitant competition. PBTS2 appeared to have the potential to positively affect the habitat fitness of PBTS1 by improving its resistance against increased concentrations of copper and penicillins. Finally, mining the chromosomes of PBTS1 and PBTS2 suggested that the bacteria could produce cytokinins, auxins, and plant growth-stimulating volatiles and that PBTS2 might interfere with ethylene levels, in support of their impact on plant development. Subsequent experimentation supported these in silico predictions. Altogether, our data provide an explanation for the observed pathogenic behavior and unveil part of the strategies used by PBTS1 and PBTS2 to interact with plants.

Deep Sequencing Reveals Early Reprogramming of Arabidopsis Root Transcriptomes Upon Ralstonia solanacearum Infection

Bacterial wilt caused by the bacterial pathogen Ralstonia solanacearum is one of the most devastating crop diseases worldwide. The molecular mechanisms controlling the early stage of R. solanacearum colonization in the root remain unknown. Aiming to better understand the mechanism of the establishment of R. solanacearum infection in root, we established four stages in the early interaction of the pathogen with Arabidopsis roots and determined the transcriptional profiles of these stages of infection. A total 2,698 genes were identified as differentially expressed genes during the initial 96 h after infection, with the majority of changes in gene expression occurring after pathogen-triggered root-hair development observed. Further analysis of differentially expressed genes indicated sequential activation of multiple hormone signaling cascades, including abscisic acid (ABA), auxin, jasmonic acid, and ethylene. Simultaneous impairment of ABA receptor genes promoted plant wilting symptoms after R. solanacearum infection but did not affect primary root growth inhibition or root-hair and lateral root formation caused by R. solanacearum. This indicated that ABA signaling positively regulates root defense to R. solanacearum. Moreover, transcriptional changes of genes involved in primary root, lateral root, and root-hair formation exhibited high temporal dynamics upon infection. Taken together, our results suggest that successful infection of R. solanacearum on roots is a highly programmed process involving in hormone crosstalk.

Virulence mechanisms of plant-pathogenic Streptomyces species: an updated review

Gram-positive Actinobacteria from the genus Streptomyces are best known for their morphological complexity and for their ability to produce numerous bioactive specialized metabolites with useful applications in human and veterinary medicine and in agriculture. In contrast, the ability to infect living plant tissues and to cause diseases of root and tuber crops such as potato common scab (CS) is a rare attribute among members of this genus. Research on the virulence mechanisms of plant-pathogenic Streptomyces spp. has revealed the importance of the thaxtomin phytotoxins as key pathogenicity determinants produced by several species. In addition, other phytotoxic specialized metabolites may contribute to the development or severity of disease caused by Streptomyces spp., along with the production of phytohormones and secreted proteins. A thorough understanding of the molecular mechanisms of plant pathogenicity will enable the development of better management procedures for controlling CS and other plant diseases caused by the Streptomyces.

Disruption and Overexpression of the Gene Encoding ACC (1-Aminocyclopropane-1-Carboxylic Acid) Deaminase in Soil-Borne Fungal Pathogen Verticillium dahliae Revealed the Role of ACC as a Potential Regulator of Virulence and Plant Defense

It has been suggested that some microorganisms, including plant growth-promoting rhizobacteria, manipulate the level of ethylene in plants by degrading 1-aminocyclopropane-1-carboxylic acid (ACC), an ethylene precursor, into α-ketobutyrate and ammonia, using ACC deaminase (ACCd). Here, we investigated whether ACCd of Verticillium dahliae, a soil-borne fungal pathogen of many important crops, is involved in causing vascular wilt disease. Overexpression of the V. dahliae gene encoding this enzyme, labeled as ACCd, significantly increased virulence in both tomato and eggplant, while disruption of ACCd reduced virulence. Both types of mutant produced more ethylene than a wild-type (70V-WT) strain, although they significantly differed in ACC content. Overexpression strains lowered ACC levels in the roots of infected plants, while the amount of ACC in the roots of plants infected with deletion mutants increased. To test the hypothesis that ACC acts as a signal for controlling defense, roots of WT and Never-ripe (Nr) tomato plants were treated with ACC before V. dahliae inoculation. Plants pretreated with ACC displayed less severe symptoms than untreated controls. Collectively, our results suggest a novel role of ACC as a regulator of both plant defense and pathogen virulence.

Enhanced stable production of ethylene in photosynthetic cyanobacterium Synechococcus elongatus PCC 7942

Ethylene is a volatile alkene which is used in large commercial scale as a precursor in plastic industry, and is currently derived from petroleum refinement. As an alternative production strategy, photoautotrophic cyanobacteria Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 have been previously evaluated as potential biotechnological hosts for producing ethylene directly from CO₂, by the over-expression of ethylene forming enzyme (efe) from Pseudomonas syringae. This work addresses various open questions related to the use of Synechococcus as the engineering target, and demonstrates long-term ethylene production at rates reaching 140 µL L⁻¹ h⁻¹ OD₇₅₀⁻¹ without loss of host vitality or capacity to produce ethylene. The results imply that the genetic instability observed earlier may be associated with the expression strategies, rather than efe over-expression, ethylene toxicity or the depletion of 2-oxoglutarate—derived cellular precursors in Synechococcus. In context with literature, this study underlines the critical differences in expression system design in the alternative hosts, and confirms Synechococcus as a suitable parallel host for further engineering.

The Role of Proteases in the Virulence of Plant Pathogenic Bacteria

A pathogenic lifestyle is inextricably linked with the constant necessity of facing various challenges exerted by the external environment (both within and outside the host). To successfully colonize the host and establish infection, pathogens have evolved sophisticated systems to combat the host defense mechanisms and also to be able to withstand adverse environmental conditions. Proteases, as crucial components of these systems, are involved in a variety of processes associated with infection. In phytopathogenic bacteria, they play important regulatory roles and modulate the expression and functioning of various virulence factors. Secretory proteases directly help avoid recognition by the plant immune systems, and contribute to the deactivation of the defense response pathways. Finally, proteases are important components of protein quality control systems, and thus enable maintaining homeostasis in stressed bacterial cells. In this review, we discuss the known protease functions and protease-regulated signaling processes associated with virulence of plant pathogenic bacteria.

Thermodynamics of Iron(II) and Substrate Binding to the Ethylene-Forming Enzyme

The ethylene-forming enzyme (EFE), like many other 2-oxoglutarate (2OG)-dependent nonheme iron(II) oxygenases, catalyzes the oxidative decarboxylation of 2OG to succinate and CO2 to generate a highly reactive iron species that hydroxylates a specific alkane C-H bond, in this case targeting l-arginine (Arg) for hydroxylation. However, the prominently observed reactivity of EFE is the transformation of 2OG into ethylene and three molecules of CO2. Crystallographic and biochemical studies have led to several proposed mechanisms for this 2-fold reactivity, but the detailed reaction steps are still obscure. Here, the thermodynamics associated with iron(II), 2OG, and Arg binding to EFE are studied using calorimetry (isothermal titration calorimetry and differential scanning calorimetry) to gain insight into how these binding equilibria organize the active site of EFE, which may have an impact on the O2 activation pathways observed in this system. Calorimetric data show that the addition of iron(II), Arg, and 2OG increases the stability over that of the apoenzyme, and there is distinctive cooperativity between substrate and cofactor binding. The energetics of binding of 2OG to Fe·EFE are consistent with a unique monodentate binding mode, which is different than the prototypical 2OG coordination mode in other 2OG-dependent oxygenases. This difference in the pre-O2 activation equilibria may be important for supporting the alternative ethylene-forming chemistry of EFE.

Comparative transcriptome analysis of the interaction between Actinidia chinensis var. chinensis and Pseudomonas syringae pv. actinidiae in absence and presence of acibenzolar-S-methyl

BACKGROUND

Since 2007, bacterial canker caused by Pseudomonas syringae pv. actinidiae (Psa) has become a pandemic disease leading to important economic losses in every country where kiwifruit is widely cultivated. Options for controlling this disease are very limited and rely primarily on the use of bactericidal compounds, such as copper, and resistance inducers. Among the latter, the most widely studied is acibenzolar-S-methyl. To elucidate the early molecular reaction of kiwifruit plants (Actinidia chinensis var. chinensis) to Psa infection and acibenzolar-S-methyl treatment, a RNA seq analysis was performed at different phases of the infection process, from the epiphytic phase to the endophytic invasion on acibenzolar-S-methyl treated and on non-treated plants. The infection process was monitored in vivo by confocal laser scanning microscopy.

RESULTS

De novo assembly of kiwifruit transcriptome revealed a total of 39,607 transcripts, of which 3360 were differentially expressed during the infection process, primarily 3 h post inoculation. The study revealed the coordinated changes of important gene functional categories such as signaling, hormonal balance and transcriptional regulation. Among the transcription factor families, AP2/ERF, MYB, Myc, bHLH, GATA, NAC, WRKY and GRAS were found differentially expressed in response to Psa infection and acibenzolar-S-methyl treatment. Finally, in plants treated with acibenzolar-S-methyl, a number of gene functions related to plant resistance, such as PR proteins, were modulated, suggesting the set-up of a more effective defense response against the pathogen. Weighted-gene coexpression network analysis confirmed these results.

CONCLUSIONS

Our work provides an in-depth description of the plant molecular reactions to Psa, it highlights the metabolic pathway related to acibenzolar-S-methyl-induced resistance and it contributes to the development of effective control strategies in open field.

Ethylene and 1-Aminocyclopropane-1-carboxylate (ACC) in Plant-Bacterial Interactions

Ethylene and its precursor 1-aminocyclopropane-1-carboxylate (ACC) actively participate in plant developmental, defense and symbiotic programs. In this sense, ethylene and ACC play a central role in the regulation of bacterial colonization (rhizospheric, endophytic, and phyllospheric) by the modulation of plant immune responses and symbiotic programs, as well as by modulating several developmental processes, such as root elongation. Plant-associated bacterial communities impact plant growth and development, both negatively (pathogens) and positively (plant-growth promoting and symbiotic bacteria). Some members of the plant-associated bacterial community possess the ability to modulate plant ACC and ethylene levels and, subsequently, modify plant defense responses, symbiotic programs and overall plant development. In this work, we review and discuss the role of ethylene and ACC in several aspects of plant-bacterial interactions. Understanding the impact of ethylene and ACC in both the plant host and its associated bacterial community is key to the development of new strategies aimed at increased plant growth and protection.

The roles of auxin during interactions between bacterial plant pathogens and their hosts

Plant pathogens have evolved several strategies to manipulate the biology of their hosts to facilitate colonization, growth to high levels in plant tissue, and production of disease. One of the less well known of these strategies is the synthesis of plant hormones and hormone analogs, and there is growing evidence that modulation of host hormone signaling is important during pathogenesis. Several plant pathogens produce the auxin indole-3-acetic acid (IAA) and/or virulence factors that modulate host auxin signaling. Auxin is well known for being involved in many aspects of plant growth and development, but recent findings have revealed that elevated IAA levels or enhanced auxin signaling can also promote disease development in some plant-pathogen interactions. In addition to stimulating plant cell growth during infection by gall-forming bacteria, auxin and auxin signaling can antagonize plant defense responses. Auxin can also act as a microbial signaling molecule to impact the biology of some pathogens directly. In this review, we summarize recent progress towards elucidating the roles that auxin production, modification of host auxin signaling, and direct effects of auxin on pathogens play during pathogenesis, with emphasis on the impacts of auxin on interactions with bacterial pathogens.

Towards engineering of hormonal crosstalk in plant immunity

Plant hormones regulate physiological responses in plants, including responses to pathogens and beneficial microbes. The last decades have provided a vast amount of evidence about the contribution of different plant hormones to plant immunity, and also of how they cooperate to orchestrate immunity activation, in a process known as hormone crosstalk. In this review we highlight the complexity of hormonal crosstalk in immunity and approaches currently being used to further understand this process, as well as perspectives to engineer hormone crosstalk for enhanced pathogen resistance and overall plant fitness.

Structural and stereoelectronic insights into oxygenase-catalyzed formation of ethylene from 2-oxoglutarate

Ethylene is important in industry and biological signaling. In plants, ethylene is produced by oxidation of 1-aminocyclopropane-1-carboxylic acid, as catalyzed by 1-aminocyclopropane-1-carboxylic acid oxidase. Bacteria catalyze ethylene production, but via the four-electron oxidation of 2-oxoglutarate to give ethylene in an arginine-dependent reaction. Crystallographic and biochemical studies on the Pseudomonas syringae ethylene-forming enzyme reveal a branched mechanism. In one branch, an apparently typical 2-oxoglutarate oxygenase reaction to give succinate, carbon dioxide, and sometimes pyrroline-5-carboxylate occurs. Alternatively, Grob-type oxidative fragmentation of a 2-oxoglutarate-derived intermediate occurs to give ethylene and carbon dioxide. Crystallographic and quantum chemical studies reveal that fragmentation to give ethylene is promoted by binding of l-arginine in a nonoxidized conformation and of 2-oxoglutarate in an unprecedented high-energy conformation that favors ethylene, relative to succinate formation.

Profiling the extended phenotype of plant pathogens: Challenges in Bacterial Molecular Plant Pathology

One of the most fundamental questions in plant pathology is what determines whether a pathogen grows within a plant? This question is frequently studied in terms of the role of elicitors and pathogenicity factors in the triggering or overcoming of host defences. However, this focus fails to address the basic question of how the environment in host tissues acts to support or restrict pathogen growth. Efforts to understand this aspect of host-pathogen interactions are commonly confounded by several issues, including the complexity of the plant environment, the artificial nature of many experimental infection systems and the fact that the physiological properties of a pathogen growing in association with a plant can be very different from the properties of the pathogen in culture. It is also important to recognize that the phenotype and evolution of pathogen and host are inextricably linked through their interactions, such that the environment experienced by a pathogen within a host, and its phenotype within the host, is a product of both its interaction with its host and its evolutionary history, including its co-evolution with host plants. As the phenotypic properties of a pathogen within a host cannot be defined in isolation from the host, it may be appropriate to think of pathogens as having an 'extended phenotype' that is the product of their genotype, host interactions and population structure within the host environment. This article reflects on the challenge of defining and studying this extended phenotype, in relation to the questions posed below, and considers how knowledge of the phenotype of pathogens in the host environment could be used to improve disease control. What determines whether a pathogen grows within a plant? What aspects of pathogen biology should be considered in describing the extended phenotype of a pathogen within a host? How can we study the extended phenotype in ways that provide insights into the phenotypic properties of pathogens during natural infections?

Transcription factor ANAC032 modulates JA/SA signalling in response to Pseudomonas syringae infection

Responses to pathogens, including host transcriptional reprogramming, require partially antagonistic signalling pathways dependent on the phytohormones salicylic (SA) and jasmonic (JA) acids. However, upstream factors modulating the interplay of these pathways are not well characterized. Here, we identify the transcription factor ANAC032 from Arabidopsis thaliana as one such regulator in response to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pst). ANAC032 directly represses MYC2 activation upon Pst attack, resulting in blockage of coronatine-mediated stomatal reopening which restricts entry of bacteria into plant tissue. Furthermore, ANAC032 activates SA signalling by repressing NIMIN1, a key negative regulator of SA-dependent defence. Finally, ANAC032 reduces expression of JA-responsive genes, including PDF1.2A Thus, ANAC032 enhances resistance to Pst by generating an orchestrated transcriptional output towards key SA- and JA-signalling genes coordinated through direct binding of ANAC032 to the MYC2, NIMIN1 and PDF1.2A promoters.

Phytohormone pathways as targets of pathogens to facilitate infection

Plants are constantly threatened by potential pathogens. In order to optimize the output of defense against pathogens with distinct lifestyles, plants depend on hormonal networks to fine-tune specific responses and regulate growth-defense tradeoffs. To counteract, pathogens have evolved various strategies to disturb hormonal homeostasis and facilitate infection. Many pathogens synthesize plant hormones; more importantly, toxins and effectors are produced to manipulate hormonal crosstalk. Accumulating evidence has shown that pathogens exert extensive effects on plant hormone pathways not only to defeat immunity, but also modify habitat structure, optimize nutrient acquisition, and facilitate pathogen dissemination. In this review, we summarize mechanisms by which a wide array of pathogens gain benefits from manipulating plant hormone pathways.

No hormone to rule them all: Interactions of plant hormones during the responses of plants to pathogens

Plant hormones are essential regulators of plant growth and immunity. In the last few decades, a vast amount of information has been obtained detailing the role of different plant hormones in immunity, and how they work together to ultimately shape the outcomes of plant pathogen interactions. Here we provide an overview on the roles of the main classes of plant hormones in the regulation of plant immunity, highlighting their metabolic and signaling pathways and how plants and pathogens utilize these pathways to activate or suppress defence.

Identification of virulence associated loci in the emerging broad host range plant pathogen Pseudomonas fuscovaginae

BACKGROUND

Pseudomonas fuscovaginae (Pfv) is an emerging plant pathogen of rice and also of other gramineae plants. It causes sheath brown rot disease in rice with symptoms that are characterized by brown lesions on the flag leaf sheath, grain discoloration and sterility. It was first isolated as a high altitude pathogen in Japan and has since been reported in several countries throughout the world. Pfv is a broad host range pathogen and very little is known about its virulence mechanisms.

RESULTS

An in planta screen of 1000 random independent Tn5 genomic mutants resulted in the isolation of nine mutants which showed altered virulence. Some of these isolates are mutated for functions which are known to be virulence associated factors in other phytopathogenic bacteria (eg. pil gene, phytotoxins and T6SS) and others might represent novel virulence loci.

CONCLUSIONS

Being an emerging pathogen worldwide, the broad host range pathogen Pfv has not yet been studied for its virulence functions. The roles of the nine loci identified in the in planta screen are discussed in relation to pathogenicity of Pfv. In summary, this article reports a first study on the virulence of this pathogen involving in planta screening studies and suggests the presence of several virulence features with known and novel functions in the Pseudomonas group of bacteria.

Comparative sequence analysis and mutagenesis of ethylene forming enzyme (EFE) 2-oxoglutarate/Fe(II)-dependent dioxygenase homologs

Background

Ethylene is one of the most used chemical monomers derived from non-renewable sources and we are investigating the possibility of producing it in yeast via the ethylene forming enzyme (EFE) from Pseudomonas syringae. To enable engineering strategies to improve the enzyme, it is necessary to identify the regions and amino acid residues involved in ethylene formation.

Results

We identified the open reading frame for the EFE homolog in Penicillium digitatum and also showed its capability of mediating ethylene production in yeast. The sequence of the EFE homologs from P.digitatum and P. syringae was compared to that of the non-functional EFE-homolog from Penicillium chrysogenum and ten amino acids were found to correlate with ethylene production. Several of these amino acid residues were found to be important for ethylene production via point mutations in P. syringae EFE. The EFE homolog from P. chrysogenum was engineered at 10 amino acid residues to mimic the P. syringae EFE, but this did not confer ethylene producing capability.

Furthermore, we predicted the structure of EFE by homology to known structures of 2-oxoglutarate/Fe(II) dependent dioxygenases. Three of the amino acids correlating with ethylene production are located in the predicted 2-oxoglutarate binding domain. A protein domain specific for the EFE-class was shown to be essential for activity. Based on the structure and alanine substitutions, it is likely that amino acids (H189, D191 and H268) are responsible for binding the Fe(II) ligand.

Conclusion

We provide further insight into the structure and function of the ethylene forming (EFE) - subclass of 2-oxoglutarate/Fe(II) dependent dioxygenases. We conclude that residues in addition to the 10 identified positions implicated in ethylene production by sequence comparison, are important for determining ethylene formation. We also demonstrate the use of an alternative EFE gene. The data from this study will provide the basis for directed protein engineering to enhance the ethylene production capability and properties of EFE.

Ethylene-forming enzyme and bioethylene production

Worldwide, ethylene is the most produced organic compound. It serves as a building block for a wide variety of plastics, textiles, and chemicals, and a process has been developed for its conversion into liquid transportation fuels. Currently, commercial ethylene production involves steam cracking of fossil fuels, and is the highest CO2-emitting process in the chemical industry. Therefore, there is great interest in developing technology for ethylene production from renewable resources including CO2 and biomass. Ethylene is produced naturally by plants and some microbes that live with plants. One of the metabolic pathways used by microbes is via an ethylene-forming enzyme (EFE), which uses α-ketoglutarate and arginine as substrates. EFE is a promising biotechnology target because the expression of a single gene is sufficient for ethylene production in the absence of toxic intermediates. Here we present the first comprehensive review and analysis of EFE, including its discovery, sequence diversity, reaction mechanism, predicted involvement in diverse metabolic modes, heterologous expression, and requirements for harvesting of bioethylene. A number of knowledge gaps and factors that limit ethylene productivity are identified, as well as strategies that could guide future research directions.

Multiple phytohormone signals control the transcriptional response to soybean aphid infestation in susceptible and resistant soybean plants

The soybean aphid (Aphis glycines) is a major phloem-feeding pest of soybean (Glycine max). A. glycines feeding can cause the diversion of photosynthates and transmission of plant viruses, resulting in significant yield losses. In this study, we used oligonucleotide microarrays to characterize the long-term transcriptional response to soybean aphid colonization of two related soybean cultivars, one with the Rag1 aphid-resistance gene and one aphid-susceptible cultivar (without Rag1). Transcriptome profiles were determined after 1 and 7 days of aphid infestation. Our results revealed a susceptible response involving hundreds of transcripts, whereas only one transcript changed in the resistant response to aphids. This nonexistent resistance response might be explained by the fact that many defense-related transcripts are constitutively expressed in resistant plants, whereas these same genes are activated in susceptible plants only during aphid infestation. Analysis of phytohormone-related transcripts in the susceptible response showed different hormone profiles for the two time points, and suggest that aphids are able to suppress hormone signals in susceptible plants. A significant activation of abscissic acid, normally associated with abiotic stress responses, at day 7, might be a decoy strategy implemented by the aphid to suppress effective salicylic acid- and jasmonate-related defenses.

Phytohormone signaling pathway analysis method for comparing hormone responses in plant-pest interactions

BACKGROUND

Phytohormones mediate plant defense responses to pests and pathogens. In particular, the hormones jasmonic acid, ethylene, salicylic acid, and abscisic acid have been shown to dictate and fine-tune defense responses, and identification of the phytohormone components of a particular defense response is commonly used to characterize it. Identification of phytohormone regulation is particularly important in transcriptome analyses. Currently there is no computational tool to determine the relative activity of these hormones that can be applied to transcriptome analyses in soybean.

FINDINGS

We developed a pathway analysis method that provides a broad measure of the activation or suppression of individual phytohormone pathways based on changes in transcript expression of pathway-related genes. The magnitude and significance of these changes are used to determine a pathway score for a phytohormone for a given comparison in a microarray experiment. Scores for individual hormones can then be compared to determine the dominant phytohormone in a given defense response. To validate this method, it was applied to publicly available data from previous microarray experiments that studied the response of soybean plants to Asian soybean rust and soybean cyst nematode. The results of the analyses for these experiments agreed with our current understanding of the role of phytohormones in these defense responses.

CONCLUSIONS

This method is useful in providing a broad measure of the relative induction and suppression of soybean phytohormones during a defense response. This method could be used as part of microarray studies that include individual transcript analysis, gene set analysis, and other methods for a comprehensive defense response characterization.

The molecular basis of host specialization in bean pathovars of Pseudomonas syringae

Biotrophic phytopathogens are typically limited to their adapted host range. In recent decades, investigations have teased apart the general molecular basis of intraspecific variation for innate immunity of plants, typically involving receptor proteins that enable perception of pathogen-associated molecular patterns or avirulence elicitors from the pathogen as triggers for defense induction. However, general consensus concerning evolutionary and molecular factors that alter host range across closely related phytopathogen isolates has been more elusive. Here, through genome comparisons and genetic manipulations, we investigate the underlying mechanisms that structure host range across closely related strains of Pseudomonas syringae isolated from different legume hosts. Although type III secretion-independent virulence factors are conserved across these three strains, we find that the presence of two genes encoding type III effectors (hopC1 and hopM1) and the absence of another (avrB2) potentially contribute to host range differences between pathovars glycinea and phaseolicola. These findings reinforce the idea that a complex genetic basis underlies host range evolution in plant pathogens. This complexity is present even in host-microbe interactions featuring relatively little divergence among both hosts and their adapted pathogens.

Dynamic evolution of pathogenicity revealed by sequencing and comparative genomics of 19 Pseudomonas syringae isolates

Closely related pathogens may differ dramatically in host range, but the molecular, genetic, and evolutionary basis for these differences remains unclear. In many Gram- negative bacteria, including the phytopathogen Pseudomonas syringae, type III effectors (TTEs) are essential for pathogenicity, instrumental in structuring host range, and exhibit wide diversity between strains. To capture the dynamic nature of virulence gene repertoires across P. syringae, we screened 11 diverse strains for novel TTE families and coupled this nearly saturating screen with the sequencing and assembly of 14 phylogenetically diverse isolates from a broad collection of diseased host plants. TTE repertoires vary dramatically in size and content across all P. syringae clades; surprisingly few TTEs are conserved and present in all strains. Those that are likely provide basal requirements for pathogenicity. We demonstrate that functional divergence within one conserved locus, hopM1, leads to dramatic differences in pathogenicity, and we demonstrate that phylogenetics-informed mutagenesis can be used to identify functionally critical residues of TTEs. The dynamism of the TTE repertoire is mirrored by diversity in pathways affecting the synthesis of secreted phytotoxins, highlighting the likely role of both types of virulence factors in determination of host range. We used these 14 draft genome sequences, plus five additional genome sequences previously reported, to identify the core genome for P. syringae and we compared this core to that of two closely related non-pathogenic pseudomonad species. These data revealed the recent acquisition of a 1 Mb megaplasmid by a sub-clade of cucumber pathogens. This megaplasmid encodes a type IV secretion system and a diverse set of unknown proteins, which dramatically increases both the genomic content of these strains and the pan-genome of the species.

Breakthroughs in genomics have unleashed a new suite of tools for studying the genetic bases of phenotypic differences across diverse bacterial isolates. Here, we analyze 19 genomes of P. syringae, a pathogen of many crop species, to reveal the genetic changes underlying differences in virulence across host plants ranging from rice to maple trees. Surprisingly, a pair of strains diverged dramatically via the acquisition of a 1 Mb megaplasmid, which constitutes roughly 14% of the genome. Novel plasmids and horizontal genetic exchange have contributed extensively to species-wide diversification. Type III effector proteins are essential for pathogenicity, exhibit wide diversity between strains and are present in distinct higher-level patterns across the species. Furthermore, we use sequence comparisons within an evolutionary context to identify functional changes in multiple virulence genes. Overall, our data provide a unique overview of evolutionary pressures within P. syringae and an important resource for the phytopathogen research community.

Transformed tobacco (Nicotiana tabacum) plants over-expressing a peroxisome proliferator-activated receptor gene from Xenopus laevis (xPPARα) show increased susceptibility to infection by virulent Pseudomonas syringae pathogens

Transgenic tobacco plants capable of over-expressing Xenopus PPARα (xPPARα), a transcription factor known to be required for peroxisome proliferation in animals, were recently generated. These plants (herewith referred to as PPAR-OE) were found to have increased peroxisome abundance, higher peroxisomal acyl-CoA oxidase and catalase activity and modified fatty acid metabolism. Further characterization of PPAR-OE plants revealed a higher susceptibility to virulent and a partial loss of resistance to avirulent Pseudomonas syringae pathogens, whereas the basal resistance response remained unaffected. Biochemical- and defense-related gene expression analyses showed that increased susceptibility to bacterial invasion coincided with the generalized reduction in H(2)O(2) and salicylic acid (SA) levels observed within the first 24 h of bacterial contact. Decreased H(2)O(2) levels were correlated with modified activity levels of catalase and other antioxidant enzymes. A correspondence between a rapid (within 1-24 hpi; ACCO and AOC) and sustained increase (up to 6 days pi; ACCO) in the expression levels of ethylene (ACCO) and jasmonic acid (AOC) biosynthetic genes and a higher susceptibility to virulent bacterial invasion was also observed in PPAR-OE plants. Conversely, no apparent differences in the short- and/or long-term expression levels of markers for the hypersensitive-response, oxidative burst and systemic-acquired resistance were observed between wild type and PPAR-OE plants. The results suggest that peroxisome proliferation could lead to increased susceptibility to bacterial pathogens in tobacco by altering the redox balance of the plant and the expression pattern of key defense signaling pathway genes.

Hormone crosstalk in plant disease and defense: more than just jasmonate-salicylate antagonism

Until recently, most studies on the role of hormones in plant-pathogen interactions focused on salicylic acid (SA), jasmonic acid (JA), and ethylene (ET). It is now clear that pathogen-induced modulation of signaling via other hormones contributes to virulence. A picture is emerging of complex crosstalk and induced hormonal changes that modulate disease and resistance, with outcomes dependent on pathogen lifestyles and the genetic constitution of the host. Recent progress has revealed intriguing similarities between hormone signaling mechanisms, with gene induction responses often achieved by derepression. Here, we report on recent advances, updating current knowledge on classical defense hormones SA, JA, and ET, and the roles of auxin, abscisic acid (ABA), cytokinins (CKs), and brassinosteroids in molding plant-pathogen interactions. We highlight an emerging theme that positive and negative regulators of these disparate hormone signaling pathways are crucial regulatory targets of hormonal crosstalk in disease and defense.

Abscisic acid-induced resistance against the brown spot pathogen Cochliobolus miyabeanus in rice involves MAP kinase-mediated repression of ethylene signaling

The plant hormone abscisic acid (ABA) is involved in an array of plant processes, including the regulation of gene expression during adaptive responses to various environmental cues. Apart from its well-established role in abiotic stress adaptation, emerging evidence indicates that ABA is also prominently involved in the regulation and integration of pathogen defense responses. Here, we demonstrate that exogenously administered ABA enhances basal resistance of rice (Oryza sativa) against the brown spot-causing ascomycete Cochliobolus miyabeanus. Microscopic analysis of early infection events in control and ABA-treated plants revealed that this ABA-inducible resistance (ABA-IR) is based on restriction of fungal progression in the mesophyll. We also show that ABA-IR does not rely on boosted expression of salicylic acid-, jasmonic acid -, or callose-dependent resistance mechanisms but, instead, requires a functional Galpha-protein. In addition, several lines of evidence are presented suggesting that ABA steers its positive effect on brown spot resistance through antagonistic cross talk with the ethylene (ET) response pathway. Exogenous ethephon application enhances susceptibility, whereas genetic disruption of ET signaling renders plants less vulnerable to C. miyabeanus attack, thereby inducing a level of resistance similar to that observed on ABA-treated wild-type plants. Moreover, ABA treatment alleviates C. miyabeanus-induced activation of the ET reporter gene EBP89, while derepression of pathogen-triggered EBP89 transcription via RNA interference-mediated knockdown of OsMPK5, an ABA-primed mitogen-activated protein kinase gene, compromises ABA-IR. Collectively, these data favor a model whereby exogenous ABA enhances resistance against C. miyabeanus at least in part by suppressing pathogen-induced ET action in an OsMPK5-dependent manner.

Biphasic ethylene production during the hypersensitive response in Arabidopsis: a window into defense priming mechanisms?

The hypersensitive response (HR) is a cell death phenomenon associated with localized resistance to pathogens. Biphasic patterns in the generation of H(2)O(2), salicylic acid and ethylene have been observed in tobacco during the early stages of the HR. These biphasic models reflect an initial elicitation by pathogen-associated molecular patterns followed by a second phase, induced by pathogen-encoded avirulence gene products. The first phase has been proposed to potentiate the second, to increase the efficacy of plant resistance to disease. This potentiation is comparable to the "priming" of plant defenses which is seen when plants display systemic resistance to disease. The events regulating the generation of the biphasic wave, or priming, remains obscure, however recently we demonstrated a key role for nitric oxide in this process in a HR occurring in tobacco. Here we use laser photoacoustic detection to demonstrate that biphasic ethylene production also occurs during a HR occurring in Arabidopsis. We suggest that ethylene emanation during the HR represents a ready means of visualising biphasic events during the HR and that exploiting the genomic resources offered by this model species will facilitate the development of a mechanistic understanding of potentiating/priming processes.

Expression of ethylene-forming enzyme (EFE) of Pseudomonas syringae pv. glycinea in Trichoderma viride

The efe gene encoding an ethylene-forming enzyme from Pseudomonas syringae pv. glycinea has been expressed for the first time under the control of Trichoderma reesei cbh1 promoter in Trichoderma viride. Reverse transcription polymerase chain reaction analysis showed that transformant Y2 produced mRNA of the efe gene. Southern blot analysis showed that there was one copy of efe gene which was integrated into the chromosomal DNA of T. viride. Ethylene production by transformant Y2 was efficiently induced by cellulose, while very low level of ethylene was produced when sodium carboxymethyl cellulose or lactose was used as carbon source. Peptone exerted a much greater stimulatory effect on ethylene production. A high level of ethylene was produced when transformant Y2 was cultured in solid fermentation medium containing wheat straw, indicating that plant wastes could be directly converted to ethylene by the recombinant filamentous fungus.

Ethylene as a modulator of disease resistance in plants

The role of ethylene in the hormonal regulation of plant development has been well established. In addition, it has been implicated in biotic stress, both as a virulence factor of fungal and bacterial pathogens and as a signaling compound in disease resistance. This apparent discrepancy has stimulated research on the effects of various types of pathogens on mutant and transgenic plants that are impaired in ethylene production or perception. It has become clear that ethylene differentially affects resistance against pathogens with different lifestyles and plays an important role in mediating different types of induced resistance.

Pseudomonas syringae pv. tomato type III effectors AvrPto and AvrPtoB promote ethylene-dependent cell death in tomato

The type III secretion system (TTSS) of Pseudomonas syringae pv. tomato (Pst) injects into the plant cell effector proteins that play an essential role in the formation of bacterial speck disease. To investigate the molecular roles of TTSS effectors in disease formation, we used a cDNA microarray to analyze the expression of approximately 8600 random tomato genes in response to wild-type Pst strain DC3000 and a mutant lacking a functional TTSS. Many of the differentially expressed genes identified encode proteins associated with hormone response or hormone biosynthesis pathways. Using isogenic mutant strains of DC3000, we monitored host transcriptional changes in response to the TTSS effector proteins AvrPto and AvrPtoB, both of which are important virulence factors on susceptible tomato lines. We found that AvrPto and AvrPtoB induce a set of host genes involved in ethylene biosynthesis and signaling, and in particular they regulate the expression of two genes, LeACO1 and LeACO2, encoding the ethylene-forming enzyme ACC oxidase. Analysis of transgenic tomato lines with diminished ACC oxidase activity revealed that ethylene production by the host is required for the full virulence activity of both AvrPto and AvrPtoB. AvrPto and AvrPtoB therefore appear to promote enhanced disease in tomato leaves, in part, by upregulating genes involved in ethylene production.

The phytotoxin coronatine and methyl jasmonate impact multiple phytohormone pathways in tomato

Coronatine (COR) is a phytotoxin produced by several pathovars of Pseudomonas syringae and consists of coronafacic acid (CFA), an analog of methyl jasmonic acid (MeJA), and coronamic acid (CMA), which resembles 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor to ethylene. An understanding of how COR functions, is perceived by different plant tissues, and the extent to which it mimics MeJA remain unclear. In this study, COR and related compounds were examined with respect to structure and function. The results indicate that conjugation of CFA to an amino acid is required for optimal activity in tomato, including chlorosis, changes in chloroplast structure, cell wall thickening, accumulation of proteinase inhibitors, induction of anthocyanins, and root growth inhibition. cDNA microarrays were utilized to understand the molecular processes that are regulated by MeJA, COR, CFA and CMA in tomato leaves. A comparison of COR- and MeJA-regulated transcriptomes revealed that COR regulated 35% of the MeJA-induced genes. There was significant overlap in the number of COR and CFA-regulated genes with CFA impacting the expression of 39.4% of the COR-regulated genes. Taken together, the results of biological assays, ultrastructural studies, and gene expression profiling demonstrate that: (1) the intact COR molecule impacts signaling in tomato via the jasmonic acid, ethylene, and auxin pathways; (2) CMA does not function as a structural analog of ACC; (3) COR has a broader range of functions than either CFA or CMA; and (4) COR and MeJA share similar, but not identical activities and impact multiple phytohormone pathways in tomato.